A stripline filter in which a stripline-type resonator is provided on a dielectric substrate, is used in various fields (e.g., see Patent Document 1).
Here, a configuration of an existing stripline filter will be described. FIG. 1 is a top perspective view of the stripline filter.
In the stripline filter 101 resonant lines 113A and 113B are formed on a top surface of a dielectric substrate 110. The resonant line 113A is a ¼ wavelength resonant line, and is connected to a ground electrode (not shown) on a bottom surface of the dielectric substrate 110 via an electrode 119A formed on the back surface in the drawing. The resonant line 113B is a ¼ wavelength resonant line, and is connected to the ground electrode (not shown) on the bottom surface of the dielectric substrate 110 via an electrode 119B formed on the front surface in the drawing. In the stripline filter 101, in order to reduce an element size, the resonant lines 113A and 113B have wide electrode parts 112A and 112B formed at edges of the substrate top surface, respectively so as to have substantially L shapes in which the resonant lines 113A and 113B are bent, whereby the lengths of the resonant lines 113A and 113B are extended.    Patent Document 1: Japanese Unexamined Utility Model Registration Application Publication No. 59-91003
In the stripline filter of the above configuration, the adjacent resonant lines are coupled to each other by causing straight portions thereof on the opposite sides of the corner portions of the L shapes to face each other. In this case, the interval between the resonant lines and the length by which the resonant lines face each other are determined in accordance with a coupling amount needed, and the resonator length of each resonant line needs to be set by the width of the wide electrode part. Thus, the element size expanded by the lengths of the wide electrode parts needs to be secured, and hence the reduction of the element size is limited.
In addition, when a plurality of filters are cut out of a single motherboard during manufacture, electrodes are formed on side surfaces after cutting of each filter. The accuracy for forming the electrodes on the side surfaces is likely to deteriorate when compared to that for forming electrodes on a top surface or a bottom surface of a dielectric substrate. Due to deviation of the electrode formed on the side surface, the width of a portion where an electrode on the top surface is connected to an electrode on the side surface, changes. Due to this change, a poor connection of the electrodes occurs or filter characteristics vary. Thus, there is a possibility that the efficiency percentage of products will be reduced.
Moreover, due to variation of the cutting position of dicing when cutting out each filter, the size of the wide electrode part of the resonant line greatly changes. Due to this, there is a possibility that the filter characteristics will vary and the efficiency percentage of products will be reduced. In addition, burring or peeling may occur at the electrode due to dicing. Due to this as well, there is a possibility that the filter characteristics will vary and the efficiency percentage of products will be reduced.